You started this post already through part of the diagnostic procedure by loosening the steel line, but you didn't say where you loosened it. You're looking for the closest place to the master cylinder where the brake fluid is being trapped and is not being allowed to flow back into the reservoir. I may have assumed incorrectly, but I thought you loosened the line right at the master cylinder. If that is wrong, let me know.
If I was right, there's only a few things that can cause the fluid to be blocked from returning, and all of them have to do with the master cylinder's lip seals being too far forward and blocking the return ports. A misadjusted brake light switch can hold the brake pedal down a little and do that. A misadjusted booster push rod on those that are adjustable can do that. The third cause is brake fluid contaminated with a petroleum product, and that is a real big deal that warrants much more discussion.
Friday, March 13th, 2015 AT 6:20 PM